Force Between Surfaces with Adsorbed Molecules

Force Between Surfaces with Adsorbed Molecules 

In Section 7.6 on thin wetting films, we discussed one link between adsorption and surface forces. There we studied the adsorption of a liquid film from its vapor onto a solid surface and related it to the force between the solid-liquid and liquid-vapor interfaces. In this section, we get to know another aspect of adsorption and surface forces. We consider the force between two solid surfaces in a liquid. If dissolved molecules in the liquid adsorb to the solid surfaces, they might significantly influence the interaction between solid surfaces. This was, for example, observed by Rehbinder et al. [1301], who observed that surface-active substances can stabilize suspensions of hydrophobic particles in aqueous medium and of hydrophilic 

When the two surfaces approach each other, adsorbed molecules are not only restricted in their conformational fiuctuations but also at some point are forced to leave the surface. To include this effect, let us consider two parallel plates separated by a distance x interacting across a solution containing different kinds (components) of dissolved molecules. These molecules are allowed to adsorb to the two surfaces. The surface excess of component i is Fj. In general, the surface excess on one surface is infiuenced by the presence of the other surface so that Fj is a function of the distance F (%). Assuming the system is in full thermodynamic equilibrium. Hall, Ash, Everett, and Radke [1304, 1305] derived a relation between the force per unit area, the amount adsorbed in mol m , and the concentration of molecules in solution c  

Equation (10.24) contains the concentration via the chemical potential p . The chemical potential of the ith component is 

Following Subramanian and Ducker [1306], we integrate Eq. (10.24) on both sides from infinite distance to a distance x. With = — J/ hc, we arrive at 

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Force Between Surfaces with Adsorbed Molecules... 

The main difference between carbon nanotubes and high surface area graphite is the curvature of the graphene sheets and the cavity inside the tube. In microporous solids with capillaries which have a width not exceeding a few molecular diameters, the potential fields from opposite walls will overlap so that the attractive force which acts upon adsorbate molecules will be increased in comparison with that on a flat carbon surface [16]. This phenomenon is the main motivation for the investigation of the interaction of hydrogen with carbon nanotubes (Figure 5.14). 

All adsorption processes result from the attraction between like and unlike molecules. For the ethanol-water example given above, the attraction between water molecules is greater than between molecules of water and ethanol As a consequence, there is a tendency for the ethanol molecules to be expelled from the bulk of the solution and to concentrate at die surface. This tendency increases with the hydrocarhon chain-length of the alcohol. Gas molecules adsorb on a solid surface because of die attraction between unlike molecules. The attraction between like and unlike molecules arises from a variety of intermolecular forces. London dispersion forces exist in all types of matter and always act as an attractive force between adjacent atoms and molecules, no matter how dissimilar they are. Many oilier attractive forces depend upon die specific chemical nature of the neighboring molecules. These include dipole interactions, the hydrogen bond and the metallic bond. 

If we consider a gas in equilibrium with adsorbed molecules on a solid surface, at sufficiently low gas pressures the adsorbed molecules will form a dilute two-dimensional system with negligible interactions. This is analogous to the perfect gas state in three dimensions. Under these conditions we are interested in the behavior of essentially independent adsorbed molecules, the only forces of interest being those between adsorbed molecules and the adsorbent. The interaction of an adsorbed molecule with the adsorbent is (in the usual approximation of additive forces) the sum of the separate interactions of the adsorbed molecule with the atoms or molecules of the adsorbent in the immediate neighborhood of the adsorbed molecule. 

We have studied the effect of bromine substituents on the two-dimensional condensation of C, U, and uridine. The potential of maximum adsorption, and thus the potential of the capacitance pit, depends on the mutual competition between the electrostatic and nonelectrostatic adsorption forces and the forces that repulse the adsorbed molecules from the electrode surface. In neutral bases and nucleosides, the nonelectrostatic adsorption usually prevails and the pit appears near the p.z.c. With C at pH 5.0, the electrostatic adsorption on the negatively charged electrode surface via the positive charge on AI-3... 

Five types of forces between colloidal particles may be identified (i) repulsive forces, from the overlap of electrical double-layers (ii) dispersion forces, from long-range van der Waals attraction between molecules in neighbouring particles (iii) steric forces, from interaction of macromolecules adsorbed at the particle surface (iv) structural and Brownian forces, from interaction with solvent molecules of the dispersion medium and (v) hydrodynamic forces. 

It is obvious that a force field is defined for polyatomic systems with well defined bonds and can only be applied to such systems. A force field type interatomic potential is potentially capable of describing all degrees of freedom of a zeolite catalyst loaded with adsorbed molecules. The internal flexibility of both the zeolite and the adsorbed molecule could be accounted for and the nonbond term would provide a description of the intermolecular interaction between the molecule and the zeolite surface. 

A large number of ordered surface structures can be produced experimentally on single-crystal surfaces, especially with adsorbates [H]. There are also many disordered surfaces. Ordering is driven by the interactions between atoms, ions or molecules in the surface region. These forces can be of various types covalent, ionic, van der Waals, etc and there can be a mix of such types of interaction, not only within a given bond, but also from bond to bond in the same surface. A surface could, for instance, consist of a bulk material with one type of internal bonding (say, ionic). It may be covered with an overlayer of molecules with a different type of intramolecular bonding (typically covalent) and the molecules may be held to the substrate by yet another fomi of bond (e.g., van der Waals). 

There are two ways a solute can interact with a stationary phase surface. The solute molecule can interact with the adsorbed solvent layer and rest on the top of it. This is called sorption interaction and occurs when the molecular forces between the solute and the stationary phase are relatively weak compared with the forces between the solvent molecules and the stationary phase. The second type is where the solute molecules displace the solvent molecules from the surface and interact directly with the stationary phase itself. This is called displacement interaction and occurs when the interactive forces between the solute molecules and the stationary phase surface are much stronger than those between the solvent molecules and the stationary phase surface. An example of sorption interaction is shown in Figure 9. 

We should mention here one of the important limitations of the singlet level theory, regardless of the closure applied. This approach may not be used when the interaction potential between a pair of fluid molecules depends on their location with respect to the surface. Several experiments and theoretical studies have pointed out the importance of surface-mediated [1,87] three-body forces between fluid particles for fluid properties at a solid surface. It is known that the depth of the van der Waals potential is significantly lower for a pair of particles located in the first adsorbed layer. In... 

In pure n-heptane or pure chloroform the solute molecules can either interact directly with the surface of the adsorbed solvent or displace the adsorbed solvent and interact directly with the silica surface. In the case of the solvent mixture the solute molecules may interact with the surface of either solvent or displace either solvent and interact directly with the silica surface or any combination of these possibilities. For example some solute molecules might displace the layer of n-heptane and interact directly with the surface. At the same time, those solute molecules striking the layer of chloroform may interact only with the chloroform and not be capable of displacing it, as the molecular forces between the chloroform and the silica gel are greater than the molecular forces between the solute and the silica gel. 

However, the assumption of molecule orientation normal to the surface is not convincing enough for this author, and it does not consist well with the results of the molecular d5mamics simulations for the alkane confined between solid walls. An example in Fig. 3 shows that the chain molecules near the wall are found mostly lying parallel, instead of normal, to the wall [6]. This means that the attractions between lubricant molecules and solid wall may readily exceed the inter-molecule forces that are supposed to hold the molecules in the normal direction. Results in Fig. 3 were obtained from simulations for liquid alkane with nonpolar molecules, but similar phenomenon was observed in computer simulations for the functional lubricant PFPE (per-fluoropolyether) adsorbed on a solid substrate [7], confirming that molecules near a solid wall lie parallel to the surface. 

It has been proposed recently [28] that static friction may result from the molecules of a third medium, such as adsorbed monolayers or liquid lubricant confined between the surfaces. The confined molecules can easily adjust or rearrange themselves to form localized structures that are conformal to both adjacent surfaces, so that they stay at the energy minimum. A finite lateral force is required to initiate motion because the energy barrier created by the substrate-medium system has to be overcome, which gives rise to a static friction depending on the interfacial substances. The model is consistent with the results of computer simulations [29], meanwhile it successfully explains the sensitivity of friction to surface film or contamination. 

The energy of an adsorbed species is the same anywhere on the surface and is independent of the presence or absence of nearby adsorbed molecules. This assumption implies that the forces between adjacent adsorbed molecules are so small as to be negligible and that the probability of adsorption onto an empty site is independent of whether or not an adjacent site is occupied. This assumption usually implies that the surface is completely uniform in an energetic sense. If one prefers to use the concept of a nonuniform surface with a limited number of active centers that are the only points at which chemisorption occurs, this is permissible if it is assumed that all these active centers have the same activity for adsorption and that the rest of the surface has none. 

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